Pop music is the greatest cultural phenomenon of the past 50 years and it was inevitable that its memorabilia – guitars, original lyrics, stage costumes and merchandise – would have a powerful impact on the world of collecting.

The Beatles provide the most sought-after items, some of which are gaining in value now most of the band members are either dead or retired. Since George Harrison’s death, the cost at auction of a programme signed by all four Beatles has almost doubled to around £5,000.

Last month, Harrison’s hand-written lyrics to “While My Guitar Gently Weeps” sold for £360,000. In July 2005, the lyrics to the even more seminal “All You Need Is Love” made £690,000. Both were sold by Cooper Auctions, a Surrey-based company that has so successfully exploited the insatiable global demand for pop memorabilia, especially anything linked to The Beatles, that it is planning to go public this year.

Alongside the music, pop inspired a parallel artistic movement, one that has been overlooked until very recently. It reached its peak in the late 1960s with the album covers and posters produced in and around the Summer of Love. The art that promoted psychedelia, with its twirling images and vibrant colours, symbolises a cultural moment in time as original and powerful as the posters of Toulouse-Lautrec evoke the music halls of Paris in the 1890s.

Sotheby’s has largely withdrawn from selling pop memorabilia, although Christie’s South Kensington and Bonhams continue to hold two sales a year. However, it is Sotheby’s that is offering for sale the most important collection of pop as art to reach the market to date. Inspirational Times consists of more than 2,000 items of rock art amassed by Peter Golding, a part-time harmonica player whose day job as a fashion designer gave him the time and money to visit creative hotspots from Paris to California in the years when youth and music were everything.

What began when he picked up a discarded poster after a protest meeting in Hyde Park in 1967 led on to a pioneering quest for original artwork inspired by the hippie dream. With no competitors, he was able to acquire seminal archives, including works by Rick Griffin, who, through his posters for the Grateful Dead and Jimi Hendrix, gave the Summer of Love its identity. Golding bought original art rather than posters and the centrepiece of the collection is the 7ft-high “Flying Eyeball” painting that Griffin produced in 1968 for a concert featuring Hendrix and British bluesman John Mayall.

This item alone is valued at more than $100,000 but Sotheby’s and Golding are intent on selling the collection complete, with a price tag of $5m. Their only concession is that they might split the larger section devoted to American bands from the British connection, which mainly revolves around punk and the early Beat scene in London and Paris. It also includes complete runs of key publications of the day such as International Times and Oz and posters for the UFO club in London where Pink Floyd played early concerts.

The hope is that a museum might buy the whole collection, which would immediately give it pre-eminence in a cultural field that can only grow in importance and popularity. There are already half a dozen interested buyers but the successful bidder will be expected to loan around a hundred items to a seminal exhibition on the Summer of Love planned for the Whitney Museum in New York this year.

The attraction of Inspirational Times is its completeness. All the key designers of the day, including Stanley Mouse, Victor Moscoso and David Singer, are represented. Included are all the 75 posters that Singer produced for legendary concert promoter Bill Graham. There are more than 200 double uncut posters and printing plates for the Family Dog series of concerts, many by Mouse and Moscoso. Every step of the creative process is represented in a poster by Randy Tuten for a New York concert by The Doors, going from the preparatory pencil sketch, through to the original art work and ends with the uncut printers’ proofs.

These names may be unfamiliar to many but they chronicled the great age of rock music. Even The Beatles squeeze in with Alan Aldridge’s signed proofs for the artwork for The Beatles: Illustrated Lyrics.

The occasional rock image with artistic merit appears on the market. The Rolling Stones are believed to be open to offers of around $250,000 for their famous “Tongue” symbol, and characters from the collage created by pop artist Peter Blake for The Beatles’ Sergeant Pepper album have also caused waves in the saleroom, but nothing as comprehensive as Inspirational Times has emerged and nothing similar can be expected. The art may be as escapist and transitory as the music but someone today is contemplating a major purchase that will either supply constant memories of a disgracefully spent youth – or prove a very shrewd investment.